This invention relates to a method and apparatus for performing calendering of a fiber web, preferably using an enclosed shoe roll.
Calendering of paper is performed in order to achieve a smooth surface of a fiber web, e.g. paper. Traditionally this is achieved by using two counter-acting rolls forming a nip within which a high pressure is applied to the paper surface in order to even out irregularities of the paper surface so as to form a smooth surface. A disadvantage by using the above mentioned method is that the high pressure acting on the web may cause excessive compaction of the web. As a result the thickness of the paper will be significantly reduced leading to relatively poor stiffness of the web after the calendering.
The above mentioned disadvantage can be reduced by using heat in combination with a relatively moderate pressure. The reason for this is that the fibers of the paper are plasticized if the temperature is sufficiently high (The temperature of plastification is normally about 170-210xc2x0 C., i.e. depending on the moisture content and the properties of the fibers.) Accordingly if a sufficiently heated roll, e.g. 250xc2x0 C., is used and sufficient heat transfer is achieved to the surface of the web passing the roll, a web may be produced having a smooth surface and relatively large thickness, which results in a much stiffer product than if a high pressure nip without heat would have been used.
For the above reason there are many applications where heat calendering is desired for the calendering process. A relatively recent problem in relation to heat calendering is the achievement of sufficient heat transfer, due to the trend towards higher and higher speed of the web. The faster the web moves through the nip the shorter time there will be for the transfer, i.e. shorter retention time. In U.S. Pat. No. 5,163,364 there is shown a method for solving the latter problem. U.S. Pat. No. 5,163,364 describes the use of an extended nip for obtaining sufficient retention time to ensure sufficient heating of the web surface during its travel through the nip. As shown in U.S. Pat. No. 5,163,364 the calendering zone is made up by a heated roll pressing from one side and an endless flexible belt which is pressed by means of a concave shoe press against the heated roll.
The endless flexible belt used in an extended nip is preferably made of a material that comprises polymers, resulting in relatively poor heat resistance, i.e. if the heat exceeds a certain temperature, normally about 80-100xc2x0 C., the flexible belt will be destroyed. Since the cost of such a belt is considerably high any over heating of the flexible belt must be avoided. This can be achieved by the paper web absorbing almost all of the heat from the heated roll. There is also heat being produced inside the shoe press unit, which is transferred to the belt, i.e. the heat energy developed due to friction between the belt and the load shoe. To achieve cooling of the belt from this heat the lubricant which is supplied between the belt and the load shoe is circulated and cooled. However, if the paper web is broken, the flexible belt could be destroyed due to overheating, since the heated roll would then act directly on the flexible belt. This problem would be even worse if an enclosed shoe roll would have been used, since the cooling of an open flexible belt is easier to achieve than in a closed roll, i.e. an enclosed shoe roll.
Another related problem is the start-up process. Normally, the jacket of an enclosed shoe-roll is not driven by itself, but by means of friction once the nip is closed. It is evident for the skilled person that, in a calender the web could be negatively affected by such a starting-up process. Furthermore, such a start-up process also presents a possible risk of overheating of the belt at the moment of the start-up, since the belt does not move during the first contact with the web within the heated nip, i.e. an extreme heat transfer to the belt will occur.
The object of the invention is to provide a process and apparatus which eliminates or at least minimizes the disadvantages mentioned above. This is achieved according to one aspect of the invention;
By a method for operating a calender, comprising a cylindrical heated roll and a flexible belt surrounding a stationary support beam, which supports at least one actuator which may urge a concave load shoe against the heated roll by means of said flexible belt to form an extended and heated nip through which a fiber web passes to be calendered, and a separating mechanism for having at least one of the rolls movable into and away from the nip characterised in that said flexible belt is a flexible tubular jacket which forms a part of an enclosed shoe roll such that the ends of said jacket have end walls mounted thereto, which end walls are rotatably mounted in relation to said support beam, and in that at least one of said end walls is driven by means of a drive arrangement, which drive arrangement may be activated to drive the end walls and thereby also the jacket independently of its position in relation to the fiber web or the heated roll.
The advantages of the present invention are several. The drive arrangement according to the invention enables opening and closing of the nip during operation without the risk of destroying the jacket due to overheating or damaging the flexible jacket, which results in cost savings and less down-time of the machine. Furthermore since the force from the drive arrangement interacts with the end walls of the enclosed shoe roll, and both end walls are rotated at the same rotational speed, the flexible jacket will not be negatively affected by the driving of the enclosed shoe roll, neither by wear on the jacket surface nor by tensional forces which otherwise might occur in the jacket itself. Moreover, by the possibility of axially displacing one end wall, the tension of the flexible jacket in an axial direction may be adjusted during operation, and thereby reducing the wear of the jacket due to local stress of the jacket in different directions.
Accordingly, the invention provides a new and improved method and apparatus for producing paper or paperboard, which also after calendering thereof has a good stiffness, thanks to the arrangements which provides for sufficient heat transfer also at very high speed of the fiber web such that the surface of the web will be plasticized and given an even surface by the use of a moderate pressure without suppressing the porous structure of the core of the fiber web.
According to further aspects related to the invention,
the drive arrangement is activated before the nip is closed in order to ensure a desired speed of the jacket at the moment of closure of the nip,
the speed of the web is measured and the speed of the jacket is synchronized with the speed of the web before it is brought into contact with it,
there is a detecting device, which detects if the paper web is broken and a control system interconnected with said detecting device in such a manner that said drive arrangement is activated if the web is broken and also at the same time that a separating mechanism is activated to move the heated roll and/or the jacket away out of contact with the other.
the speed of the web exceeds 600 m/min, preferably exceeds 800 m/min, and more preferably exceeds 1000 m/min, but is less than 4000 m/min,
the web being produced is paper whereby the speed of the web exceeds 1000 m/min, preferably exceeds 1500 m/min, and more preferably exceeds 1800 m/min,
the temperature of the surface of said heated roll is between 150-350xc2x0 C., preferably exceeds 170xc2x0 C., and more preferably is about 200-250xc2x0 C.,
the linear load within the nip is 100-500 kN/m, preferably less than 400 kN/m, and more preferably about 320-380 kN/m,
the linear maximum pressure within the nip is between 3-15 Mpa, preferably less than 13 Mpa, and more preferably about 8-12 Mpa,
the force transmission from the drive arrangement to said force transmitting device is achieved by means of friction,
the force transmission from the drive arrangement to said force transmitting device is achieved by means of a positively gripping drive arrangement,
the end walls are axially displaceable such that the position and tension of the flexible jacket may be controlled also during operation of the enclosed shoe roll,
a separating mechanism is arranged to the heated roll to move it out of contact with the jacket,
a separating mechanism comprises a pivoting structure having at least one lever arm pivoting about an axis, which lever arm preferably is moved by means of an hydraulic assembly,
the last step of achieving the nip involves urging the jacket out and above its unloaded position by means of the load shoe to press against the heated roll,
a separating mechanism is achieved by moving the jacket out of contact with the heated roll by means of unloading the load shoe.
These and further aspects of the invention and the advantages with the invention will become apparent from the detailed description and from the attached claims.